For the purpose of preventing adhesion or bonding between a substrate (e.g., paper, a plastic film or the like) and a pressure-sensitive adhesive material, it has been a practice to impart releasing properties by way of forming a cured coating of a silicone composition on the substrate surface. Such a product is generally called “release paper”.
Among the aforementioned pressure-sensitive adhesive materials, a silicone adhesive containing an organopolysiloxane as the main component is used in a broad range of applications because of its excellent properties in terms of heat resistance, cold resistance, chemical resistance, electrical insulation, low toxicity and the like. Since the adhesion of the silicone adhesive is markedly strong, an excellent releasability of the cured silicone coating formed on the aforementioned substrate is required, in order to easily release a pressure-sensitive adhesive tape or label from the aforementioned substrate.
The silicone compositions capable of providing a cured silicone coating having an excellent releasability which have been proposed include (1) a curable coating composition containing (A) an organopolysiloxane having a perfluoroalkyl group represented by the formula CnF2n+1CH2CH2— (n is an integer of 1 or more) and an alkenyl group, (B) a hydrosilylation catalyst containing platinum, and (C) an organohydroxypolysiloxane cross-linking agent (cf. JP-B-5-7434), and (2) a curable silicone composition containing (a) an organopolysiloxane having a perfluoropolyether group represented by the formula [CF(CF3)CF2O]nCF(CF3)CF2OCH2CH2CH2— (n is an integer of from 1 to 5) and an alkenyl group, (b) an organohydrogenpolysiloxane, and (c) an addition reaction catalyst (cf. JP-B-4-76391).
These organopolysiloxane compositions are applied to a substrate after diluting them with solvents. A fluorine solvent is suitably used as the solvent in view of the solubility of the fluorine-containing organopolysiloxane composition in the solvent.
Although fluorine solvents can sufficiently dilute the fluorine-containing organopolysiloxane, they are expensive and have a problem in that they exert adverse influences upon the natural environment when diffused into the air.
As a silicone composition which can be diluted with non-fluorine solvents and which is capable of providing a cured silicone coating excellent in the release ability, it has been proposed a silicone composition or a realse agent, which contains (A) an organopolysiloxane having at least two alkenyl groups bonded to a silicon atom and at least one fluorine-containing substituent bonded to a silicon atom, in one molecule, and having a fluorine content of from 20 to 40% by weight, (B) an organohydrogenpolysiloxane having at least three hydrogen atoms bonded to a silicon atom in one molecule, and (C) a platinum group metal catalyst (cf. JP-A-7-18185).
When this silicone composition for a release agent is dissolved in a non-fluorine solvent, it is homogeneously dissolved and transparent in appearance. However, the solution is easy to show foaming due to a slight difference in compatibility. Particularly, it frequently foams by revolution of a roll coater in a coating solution, so that pin holes and repelling phenomenon are generated on the coating surface, which is a reason for partially showing tight release.
References:
JP-B-5-7434
JP-B-4-76391
JP-A-7-18185